The invention relates to a drive system for chain sprockets of chain drives, preferably for driving chain scraper conveyors or chain-drawn ploughs for underground mining, with a drive assembly formed of an asynchronous motor and a gear mechanism, the gear mechanism being designed as an overload and load equalisation gear unit having a controllable multiple-disk clutch for overload equalisation with which multiple-disk clutch the force flow between the asynchronous motor and the chain sprocket can be disconnected.
A drive system of this type is known from DE 40 24 830 A1 or U.S. Pat. No. 5,551,902 for example. In the known chain drive, unregulated three phase a.c. asynchronous motors are used. With the downstream arranged overload and load compensation gearing, during operation of the drive system load compensation between the main drive and the auxiliary drive of the coal plough or chain scraper conveyor is brought about in order to optimise the operation of the coal plough or chain scraper conveyor and to prevent unfavourable loading conditions for the chain. In the case of the drive system in DE 40 24 830 A1 an auxiliary motor is assigned to the overload and load sharing gearing which can be switched on and off when the asynchronous motor is at a standstill in order to tension the chain. At the same time, in order to tension the chain in the drive connection between the asynchronous motor and the overload and load compensation gear mechanism, a blocking device which blocks the asynchronous motor from turning is provided.
In place of unregulated three phase a.c. asynchronous motors (rotary current asynchronous motors), pole reversing asynchronous motors (DE 37 41 762 A1) and computer-controlled three phase a.c. asynchronous motors (U.S. Pat. No. 6,008,605) have been proposed, in which for each motor the individual torque-revolution characteristic curve is stored in an assigned computer. By means of a revolution counter, when the motors are in operation the current revolutions are permanently determined in a potential-separated manner in order to regulate the motor by way of comparing the current revolutions with the individual revolutions according to the characteristic curve. Due to the proportionality of revolutions and torque, with appropriate rotary current asynchronous electric motors the given torque can be adjusted.
In underground mining, efforts are being made to use three phase a.c. motors with frequency converters, known as frequency converter motors, as electric drives. With frequency converter motors constant adjustment of the revolutions is possible. The rough underground ambient conditions, with dust, moisture and corrosion, as well as the statically determined coupling of the frequency converter motor with the overload gearing cause problems for the use of frequency converter motors which have hitherto prevented the broad possibilities of using frequency converter motors. However, one use of frequency converter motors is known in which a rotary elastic and breakthrough-proof claw coupling is arranged between the overload coupling and the frequency converter motor. The intermediate claw coupling considerably increases the space required for the drives in the underground working faces so that correspondingly assembled drive system can only be used in an underground working face if there is sufficient space available. Furthermore, when using frequency converters on chain drives there are still considerable difficulties in achieving the breakaway effect required for starting a loaded chain scraper conveyor or releasing a plough jammed in the working face.